Noise in general, and tonal noise in particular is very annoying. Low-frequency noise is very penetrating, travels very long distances and is difficult to attenuate using traditional passive control measures.
Passive noise control technology, which usually involves using absorptive materials or noise partitions, enclosures, barriers and silencers, can be bulky, ineffective and rather expensive at low frequencies. Active Noise Control (ANC), on the other hand, can be very efficient and relatively cheaper in reducing low-frequency noise.
Active Noise Control (ANC) is a technology using noise to reduce noise. It is based on the principle of superposition of sound waves. Generally, sound is a wave, which is traveling in space. If another, second sound wave having the same amplitude but opposite phase to the first sound wave can be created, the first wave can be totally cancelled. The second sound wave is named “anti-noise”.